The violet chip on a forecast marks 11 and everything above it. An 11 and a 16 share that chip. The higher integer is a faster burn rate: the index counts erythemally weighted UV, and that count keeps climbing after the color has gone violet.

11+ is a range

WHO’s practical guide prints five bands. Extreme is 11+. ICNIRP, a co-author of the index with WHO, WMO, and UNEP, says the values run from zero to over 20.

A forecast office may print the category, the whole number, or both. WHO wants that number as a single integer. The color is the shortcut laid on top. EPA’s US scale page currently puts 8 and above in one “Very High to Extreme” block, one extra-protection paragraph for the lot. The dose is still the integer.

WHO sets one index point at 0.025 W/m² of erythemally weighted UV. One Standard Erythemal Dose is 100 J/m² of the same quantity. Hold the index still for an hour and each point is about 0.9 SED:

0.025 W/m² × 3,600 s = 90 J/m².

• One hour at a steady 11 → 9.9 SED
• One hour at a steady 16 → 14.4 SED

Sixteen over eleven is about 45% more weighted UV in the same hour. ICNIRP’s indicative unadapted MED for phototypes I–II is about 2 SED—a just-perceptible reddening, not an allowance. The hour at 11 is about five of those units. The hour at 16 is about seven.

Minutes inside the bin

ARPANSA’s occupational standard lists how quickly unprotected skin passes an exposure limit of 30 J/m² in a workday. Table 4 keeps going after the label has turned Extreme:

• UV 12 — 7 minutes to the limit
• UV 14 — 6 minutes
• UV 16 — 4 minutes

At a clear-sky UV 12, the same standard puts erythema for unprotected fair skin at about 11 minutes, citing Gies and Wright 2003. WHO’s guide tells communicators to leave burn times out of the public message. People hear a safe window. Use the table as a ratio: the clock they print runs from 7 minutes at 12 to 4 minutes at 16.

Those teens are measured summers. Gies and colleagues, reporting ARPANSA data for 1996–2000, found Australian summer values regularly above 12–14, with more northerly sites occasionally reaching 16 and 17. Cancer Council Australia still cites that range. A reading near 43 is a different page: the highest UV index on record, a high-Andes instrument day.

The label flattens the day

1. "It's already Extreme, so the digits are decoration." The kit does not grow a new item at 14. WHO’s guide says that once protection is required, use clothing, sunglasses, shade, and sunscreen together, and that more of that protection is needed as the index rises. The warning is said again from 8 up. Two hours at a steady 16 is 28.8 SED. Two hours at a steady 11 is 19.8 SED. About 9 SED more—four and a half of those 2 SED units—under a label that never changed.
2. "8+ means the day is maxed out." An 8 and a 16 share EPA’s extra-protection line. Hours at UV 4 can outdose minutes at UV 8 compares a long moderate block with a short spike. Here the spike has two speeds.
3. "I'll stay as long as I stayed at 11." Phototype still sets whose minutes run out—see skin type changes burn minutes, not the index. Sunscreen does not close the gap between 11 and 16. WHO: sunscreen should never be used to prolong the duration of sun exposure.

Read the digits

Use the live UV index for the integer—current, today’s peak, the next few hours.

1. If the chip says Extreme, read the digits beside it. 11 and 16 are the same color and different rates.
2. Keep one kit from 3 up. Shade, a shirt, a brim, eye cover, sunscreen on what is left. What UV index requires sunscreen is that floor. From 8 up, repeat the warning and shorten the block. A lunch sized for 11 runs long at 16.
3. Leave the ARPANSA minutes as a speed check. They show how fast an occupational limit arrives on bare skin. They are not time you may stay uncovered.
4. Recheck through the afternoon. The chip can stay violet while the number falls from the mid-teens toward 11. That fall is dose you skipped. The late-morning climb is dose still ahead of you.

A violet square is the flag. The number next to it is the speed.

Sources

WHO categories (11+ Extreme), integer reporting, protection from 3, a repeated warning from 8, more protection at higher values, and sunscreen never used to prolong exposure: Global Solar UV Index: a practical guide. Values from zero to over 20: ICNIRP UV Index. UVI 1 = 0.025 W/m² (k_er = 40 m²/W): the WHO guide. One SED = 100 J/m²: CIE e-ILV. Unadapted MED about 2 SED for phototypes I–II: ICNIRP, Protecting Workers from Ultraviolet Radiation. Limit 30 J/m²; Tmax 7 / 6 / 4 minutes at UV 12 / 14 / 16; about 11 minutes to erythema at UV 12: ARPANSA RPS 12, Table 4 (erythema line cites Gies and Wright 2003). Australian summer peaks regularly above 12–14, occasionally 16–17 farther north: Gies et al. 2004, cited by Cancer Council Australia. EPA’s 8+ block: UV Index Scale. Steady-index arithmetic on open sky. Shade, posture, cloud, and skin move the dose. Those minutes are not a personal timer.